Question

Describe the difference between making a class a member of another class (object aggregation \(),\) and making a class a friend of another class.

Short answer

Answer: The main difference between object aggregation and friend class is that object aggregation is a relationship where one class is composed of another class's objects, implying a "has-a" relationship, while friend class allows one class to access another class's private and protected members without any specific relationship.

Step-by-step solution

Introduction to Object Aggregation

Object aggregation is a relationship between two classes, where one class is a member of another class. The word "aggregation" refers to the process of assembling multiple pieces together. In object-oriented programming, this means combining objects from distinct classes to form a new object. This relationship implies that an object of the aggregated class can only exist in the context of an object of the containing class. For example, consider a class "Engine" and another class "Car." A car must have an engine to function, so the "Engine" class can be a member of the "Car" class.

Introduction to Friend Class

A friend class is a relationship between two classes, where one class has access to the private and protected members of another class. In this case, the second class allows the access by declaring the first class as its friend. Declaring a class as a friend does not make it a subclass or a member of the other class. It's just a way of granting access to the other class's private and protected members. For example, consider a class "Car" with private attributes and protected methods. If another class "Garage" needs access to those private attributes and protected methods, the "Car" class can declare the "Garage" class as its friend.

Key Differences

Here are some key differences between object aggregation and friend class: 1. Object aggregation is a relationship where one class is composed of another class's objects, while friend class allows one class to access another class's private and protected members. 2. Object aggregation implies a "has-a" relationship between the classes, while friend class doesn't imply any specific relationship. 3. In object aggregation, the lifecycle of the aggregated objects may depend on the containing class's objects. In contrast, friend classes don't have any influence on each other's lifecycles. 4. Object aggregation leads to a more tight coupling between the classes as they become dependent on each other, while friend classes have a looser coupling, as they only grant specific access.

Examples

Here are examples to illustrate object aggregation and friend class: // Object Aggregation Example class Engine { int power; public: Engine(int p) : power(p) {} }; class Car { Engine engine; public: Car(int power) : engine(power) {} }; // Friend Class Example class Car { friend class Garage; private: int speed; protected: void accelerate() { speed += 5; } }; class Garage { public: void tuneCar(Car &car) { car.accelerate(); // accessible because Garage is a friend of Car car.speed -= 1; // also accessible because Garage is a friend of Car } };